In manufacturing of optical components, lenses, molds, and the like, preliminary operations, such as grinding or diamond turning, are performed to generate an optical surface on a raw blank of material. The preliminary operations provide the general form of the component, but leave surface defects that include turning grooves, cutter marks, and sub-surface damage. A final polishing step is required to remove these surface and sub-surface defects. Polishing is accomplished in a variety of ways depending upon the material and the surface's form (i.e.: a surface can have plano, spherical, or aspherical form).
Plano and spherical surfaces are typically polished using “full-aperture” or “full-surface” tools. Full aperture tools tend to cover over 80% of the work piece surface during polishing. Full-aperture tools may be constructed in a variety of ways, including traditional “pitch” and more recent pad-type. “Pitch” polishing tools are comprised of a soft flow-able material, such as pitch or bees wax, which is used to create a mold of the optical surface. Referring to FIG. 1, this mold is a mirror replica of the work piece surface and becomes a polishing tool 300 once the mold is modified with grooves 305. Polishing tool 300 has a support surface 310 and is fixedly attached to a shank 315 that forms an arbor 320 that is used to hold the polishing tool 300 in application. During polishing, polishing tool 300 is held against the work piece (not shown, but conventionally, made of optical glass) with an applied force and the two components are moved relative to one another in the presence of a free abrasive polishing compound, such as cerium oxide, to achieve polishing.
A pad-type full-aperture polishing tool depicted in FIG. 2 consists of a polishing tool 300 incorporated with polishing pad 325 resting or adhered to support surface 310. The polishing pad 325 is typically attached to the support surface 310 via adhesive or via friction grip as disclosed in U.S. Pat. No. 4,274,232 issued to Wylde, on Jun. 23, 1981.
Polishing of aspheric surfaces using full-aperture tools involves much iteration to rebuild or reshape the polishing tool slowing the polishing process considerably. Therefore, polishing of aspheric surfaces is commonly restricted to sub-aperture methods using ring-tools or small-area tools. Sub-aperture methods using ring-tools or small-area tools rely on a polishing tool that contacts less than 50% of the work piece surface at one time. Ring tools, as disclosed in U.S. Pat. No. 4,768,308 issued to Atkinson, III et al. on Sep. 6, 1988, have a diameter that is comparable to or larger than the radius of the work piece and contact the work piece surface over an area that is much larger than that for a small-area tool. Small-area tools contact only a small area of the work surface at a time and create an interfacial contact area that is on the order of 99% smaller than the area of the work piece surface.
Traditionally, manufacturers made polishing tools rotationally symmetric, with minimal radial and axial run-out, such as the full-aperture and sub-aperture polishing tools depicted in U.S. Pat. No. 6,033,449, issued to Cooper et al., on Mar. 7, 2000. Sub-aperture small-area tools may be outfitted with a variety of polishing head shapes, including spherical (as shown in FIG. 3), but may also include conical, cylindrical, and flat along with a polishing pad. In FIG. 3, a sub-aperture polishing tool 330 includes an arbor 335 fixedly attached to a spherical polishing head 340. It should be noted that the spherical polishing head 340 may be substituted with one of the aforementioned polishing heads of a different geometrical shape. Sub-aperture ring-tools may be considered a variation on the small-area tool with the polishing head being of ring-shaped configuration with surface contact during polishing being from 3% to 50% of the work piece surface.
Such rotationally symmetric polishing tools, as described above, require a driving device to impart various motions, for example, rotational and oscillatory motions. However, where the work piece surface has a consistent rotational motion relevant to the rotational polishing tool, unwanted grooves can occur. These unwanted grooves negatively affect the optical properties of the work piece surface, because they prevent the work piece surface from being perfectly smooth.
Driving devices, as noted in U.S. Pat. No. 1,422,505 issued to Weaver on Jul. 11, 1922, and U.S. Pat. No. 3,156,073 issued to Strasbaugh on Nov. 10, 1964, are limited in velocity and subsequent oscillation frequency due to the mass and complexity required to impart such motions. Moreover, these prior art solutions are only applicable to full aperture polishing found in spheres and plano type surfaces and not aspheric surfaces. Consequently, there is a need for a polishing tool that will effectively polish aspheric surfaces.